Temporary Backup Pump for Cooldown System

[brad4pumps]

I have been asked to design a temporary pumping system as a backup to a cooldown system. The proposed temporary system would connect two (2) diesel-driven pumps to a discharge header. The normal system header pumps deliver 5,000 GPM at 95 PSI. The proposed backup system would run 24/7 for 1 - 3 weeks.

The operating pressure on the backup pumps need to be below normal system head pressure. When the normal system header pressure falls below 80 PSI, the proposed pumps need to provide a minimum of 4,000 GPM at 80 PSI (static lift is 30' and total discharge pipe run is less than 50').

The system header pressure and flow can not drop below 4,000 GPM or 80 PSI for more than 3 seconds.

Any suggestions would be greatly appreciated. Thank You.

 

[checman]

brad4pumps, 30 feet static lift on the suction needs to be looked at. Unless you are below sea level you may be a little short on NPSHA.

Regards checman

 

[brad4pumps]

The static suction lift is 15' and the static discharge lift is 15' for a total static lift of 30'.

Thanks.

 

[Montemayor]

Static Discharge Lift?

This implies you're discharging below atmospheric pressure. Is this your discription of a pump's characteristic or someone else's?

 

[checman]

brad4pumps, Not knowing why or how fast the header would loose pressure makes this all guess work. What is the allowable overlap for the back up pumps? I cannot see the diesel engines getting a signal and dependably starting and coming up to speed in three seconds every time. Also you would want to limit the starts and stops. If the back-up pump was sized so that its recommended minimum flow was at 95 psi you could start it at 82 psi and shut it off at 95 psi. Or something along that line. Another idea would be to incorporate a large accumulator and run the back up pumps off of its level. With that you might have the normal accumulator level to be 3/4 full at 95 psi (call this 100%). The pump would come on when the accumulator level dropped to 25% of normal and off when it hit 50%. This is a bit more complicated than it sounds. Other things to consider would be back flow through the back-up pump. Slamming check valves when shutting the back-up pumps down when the primary pumps are running, It would take to much time to go into all the possible ways of addressing this application or its possible problems. I would be interested in see what you have in mind.

Regards

checman

 

[brad4pumps]

Thanks for your input, please let me explain the process further. Normally, there are three (3) cooldown pumps used to cool down the process equipment with an elevated storage tank used as a backup should the power fail. If the power fails, the elevated tank gravity cools the process for a period of 18 minutes. The elevated tank needs to be taken out of service for repair. That is why the diesel pumps are needed.

While the elevated tank is out of service (estimated at 1 - 3 weeks), the diesel pumps are required to be running 24/7 but not overcoming the normal header pressure. Should the power fail and the normal header pressure drops, the diesel pumps become the primary cooling pumps and are required to provide a minimum of 4000 GPM at 80 PSI. The flow and pressure can not drop below these values for more than 3 seconds.

The pumps we are looking at have a minimum flow of 2000 GPM. We realize that starting the pumps and getting up to full speed within 3 seconds is not an option. We have considered using a PID loop control with a pump discharge pressure setpoint of 85 PSI. With normal header pressure of 95 PSI the diesel pumps would recirculate back into the suction pit through an electric-actuated valve fitted with a fail to close option (I have been told this is available). If the power fails the recirculation valve begins to close and the pump would speed up to keep the 85 PSI setpoint. The cycle time on a 4" valve is 20 seconds and the velocity through a 4" valve at 2000 GPM is 51 ft/s. We are concerned about the 3 second time constraint as well as the velocity through the valve.

We are still trying to come up with a viable solution. Any thoughts or guidance would be greatly appreciated. Thank You.

 

[checman]

brad4pumps, You could just make a discharge loop on the back up pumps by putting a back pressure valve on the discharge before the pond. . Tie the process loop and the back up loop together with a check valve and another back pressure valve (to keep the pump on its curve). The pressure on the process loop will keep the check valve closed until the backup loop pressure exceeds the process loop. You might need the pressure on the back up pump loop to be slightly higher to counter the closing effect of the process side.

On the 51 feet per second you will need ear plugs. What is the choke flow for that valve? I would not count on that valve.
Regards
checman

 

[unclesyd]

Could you take some of the primary pumps off line and bring the diesel pumps online to fill in when the water tower is to be taken out of service. In other words the diesels would be online in the event of a power failure.

I don't know the numbers or details but here is ours as I recall.

We have a similar situation as yours where a water tower backs up a process cooling, quench and fire water system. We have both diesel and steam turbine pumps as backup to the system. If the water tower is to be taken out of service some of the primary pumps are taken offline and the standby pumps are brought online to replace the lost capacity of the primaries. The suction of the standby pumps are normally online with the primaries but in case of a fire they can draw on two 1 million gallon potable water storage tanks or in a real crisis they switch to river water. Our electric supply normally has 3 discrete sources and at times 4, but the system has been used 3 times in tthe 50 year history of the plant.

 

[brad4pumps]

Thanks for the input. The back pressure valve on the discharge loop is a great idea. I prefer to use the diesel back up pumps as the primary pumps during the water tower service period as suggested by unclesyd. I believe that will be the simplest as well as the most reliable. Thanks again for all of your input. Have a great day!